[typing] allow Field to have a default of None

Previously we used None as the sentinel for determining that a user had
not specified a default value for a field. This meant that a user could
never select `None` as the default value. We now properly use UNSET to
determine if a default was passed, this aligns better with the code as
it stands given that if no default is passed we'll use UNSET anyway.

disco/types/base.py

@@ -55,7 +55,7 @@ class ConversionError(Exception):
 
 
 class Field(object):
-    def __init__(self, value_type, alias=None, default=None, create=True, ignore_dump=None, cast=None, **kwargs):
+    def __init__(self, value_type, alias=None, default=UNSET, create=True, ignore_dump=None, cast=None, **kwargs):
         # TODO: fix default bullshit
         self.true_type = value_type
         self.src_name = alias
@@ -64,21 +64,23 @@ class Field(object):
         self.cast = cast
         self.metadata = kwargs
 
-        if default is not None:
+        # Only set the default value if we where given one
+        if default is not UNSET:
             self.default = default
+        # Attempt to use the instances default type (e.g. from a subclass)
         elif not hasattr(self, 'default'):
-            self.default = None
+            self.default = UNSET
 
         self.deserializer = None
 
         if value_type:
             self.deserializer = self.type_to_deserializer(value_type)
 
-            if isinstance(self.deserializer, Field) and self.default is None:
+            if isinstance(self.deserializer, Field) and self.default is UNSET:
                 self.default = self.deserializer.default
             elif (inspect.isclass(self.deserializer) and
                     issubclass(self.deserializer, Model) and
-                    self.default is None and create):
+                    self.default is UNSET and create):
                 self.default = self.deserializer
 
     @property
@@ -94,7 +96,7 @@ class Field(object):
             self.src_name = name
 
     def has_default(self):
-        return self.default is not None
+        return self.default is not UNSET
 
     def try_convert(self, raw, client, **kwargs):
         try:

tests/types/base.py

@@ -29,3 +29,13 @@ def test_cached_property_clear_on_update(model):
     assert inst.value == 4
     inst.inplace_update(model(a=2, b=3))
     assert inst.value == 5
+
+
+def test_defaults():
+    class TestModel(Model):
+        a = Field(int, default=None)
+        b = Field(int, default=0)
+
+    model = TestModel()
+    assert model.a is None
+    assert model.b == 0